The present invention relates to a vacuum interrupter and more particularly to an improved electrode structure for a vacuum interrupter. Still more particularly, the invention relates to an improved electrical connector and main electrode structure forming a part of the electrodes for a vacuum interrupter.
A vacuum interrupter for handling a high current generally includes a pair of main electrodes disposed in a vacuum vessel so that at least one of the pair is movable toward and away from the other, coil conductors mounted on the rear surfaces of the main electrodes, and conductor rods extending to the exterior of the vacuum vessel from the rear surfaces of the coil conductors. Current flows from one of the conductor rods to the other through the coil conductors and main electrodes. When one of the conductor rods is urged by an actuator for the purpose of interrupting the current, at least one of the main electrodes is moved away from the other, and an arc current is caused to flow between the spaced electrodes. This arc current is dispersed into a plurality of filament-like arc currents by a magnetic field created by the flow current through the coil conductors.
Electrical connectors provide a bridge for current flow between the main electrode and the coil conductor. When the main electrodes are in the closed position of contact and in contact with each other, current flows through the points of contact between the main electrodes. Thus, current flows through the main electrode between path defined by the electrical connectors and the points of contact. Because the points of contact between the main electrodes varies with the construction of each main electrode, it is impossible to predict the correct path through the main electrode. Preferably, the current will flow through the periphery of the min electrode, when the distance to the electrical connector is shortest and thus where ohmic resistance is lowest. However, in many situations, the point of contact will be in the middle of the main electrodes, where the distance and the electrical connector is greatest and ohmic resistance consequently is much higher. As resistance increases the likelihood of thermal breakdown also increases. Oftentimes the thermal breakdown results in the main electrodes welding together at their middle. Once the main electrodes weld together, it becomes extremely difficult to separate them.
U.S. Pat. No. 3,946,179 discloses a coil conductor that comprises a plurality of conductive arms connected to arcuate sections. The arms connect at one end to a conductor rod and diverge in a generally radial direction therefrom to connect to an arcuate section at the other end. The arcuate sections extend circumferentially from the arms and connect to a main electrode. A plurality of arms and associated arcuate sections with clearances formed between adjacent arcuate sections, form an imaginary coil of one turn. Current flows from the rod to the main electrode through the spaced arms and associated arcuate sections. The one-turn current produces a uniform axial magnetic field that produces the diffuse, filamentary arc currents between the main electrodes.
The use of the clearance in U.S. Pat. No. 3,946,179 to produce the coil effect in the coil conductor results in a weak axial magnetic field in the region of the clearances. Arc currents have a tendency to migrate from a low intensity region toward a high intensity region of an axial magnetic field. Thus, the arc current flowing into the main electrode migrates away from the region of the clearances, causing localized overheating of the main electrode. Because the entire area of the main electrode cannot be utilized effectively for the current interruption, it becomes necessary to increase the size of the main electrode. In addition, there is no provision to maintain current in the periphery of the main electrodes when they are in contact, increasing the possibility that current will flow through the center of the main electrode.
In commonly assigned U.S. Pat. No. 4,837,481, a uniform axial magnetic field is produced by providing parallel slits in the coil conductors. However, the configuration of the electrical connectors still provide certain limitations in the life of the main electrode. Although greatly improving over the prior art, the arc current still is not maintained consistently around the periphery of the main electrode when the main electrodes are in contact. In U.S. Pat. No. 4,839,481, current passes from the coil conductor to the main electrode through electrical connectors. When the main electrodes are in the closed position and in contact with each other the contact resistance depend on the location of the points of contact between the main electrodes. If the points of contact are in the center of the main electrodes the current will be forced through a region of relatively high resistance, causing the temperature of the main electrode to increase, until thermal breakdown results. In a worst case scenario, the main electrodes weld together and cannot be separated.
In commonly assigned U.S. Pat. No. 4,871,808, incorporated herein by reference, a uniform axial magnetic field is maximized, thereby providing an arc current that is more evenly distributed. The axial magnetic field is maximized by reducing the radial magnetic field through the use of a uniform cylindrical coil conductor. In addition, the invention disclosed a structure support rod to reduce mechanical stress. However, as in U.S. Pat. No. 4,839,471, current is forced through the points of contact between the main electrodes. As a result, current tends to flow in the center of the main electrodes, thereby increasing the resistance of the main electrodes and shortening the life of the main electrodes.